The Line 2 Checksum is determined by adding all the previous numbers in Line 2 and taking the last digit in the final sum. All letters, periods and plus signs are taken as "0". Negative signs are taken as "1".

This is mainly used to verify the first line's authenticity, or its successful transmission to another party.

For the ISS TLE - Line 2:

2+2+5+5+4+4+0+5+1+.(0)+6+4+2+1+0+6+3+.(0)+2+7+3+4+0+0+0+7+4+1+5+3+0+8+.(0)+6+2+6+3+2+4+9+.(0)+9+1+7+
7+1+5+.(0)+7+4+6+6+8+6+0+0+4+1+4+9+0 = 211

Taking the last digit gives a Line 2 Checksum of 1, which matches the final digit in Line 2.